A superemissive material can generally be described as a material that, when heated to a threshold temperature, includes one or more electrons that jump to a different electron energy level in quantum increments which causes the emission of visible or infrared radiation in a wavelength band related to the electron's inner electron shell vacancy. Emitted radiation produced as a result of such electron transition is often within a narrow band and can, therefore, be absorbed efficiently by a photovoltaic device, such as silicon cell, indium-gallium-arsenic cell, or the like to produce an output voltage and current. A superemission formed from such thermally-stimulated superemissive material produces radiation in relatively concentrated narrow spectral bands compared to blackbody or greybody emitters which typically exhibit a broadband thermal emission. As a result of the concentrated, narrow spectral band, photons emitted from the superemitter and focused to the photovoltaic power system have greater efficiency than that generated by a blackbody emitter operating at the same heat flux. However, a blackbody emitter may be constructed to emit visible or infrared radiation in a narrow spectral band by using a band-pass filter interposed between the superemitter and the photovoltaic device. Filtering, however, is not energy efficient.
Photon generators that use superemitters to emit radiation and, therefore, generate photons are well-known and are disclosed in U.S. Pat. Nos. 4,776,895, 4,793,799 and 4,906,178 and are also disclosed in U.S. patent application Ser. No. 08/085,117. For example, such a photon generator includes a porous ceramic burner in the shape of a cylinder having an annular passage extending therethrough. A superemitting fiber layer is disposed along the inside wall of the ceramic burner and is made from a high temperature fiber or coating comprising, for example, pure or doped oxides of uranium, thorium, ytterbium, aluminum, gallium, yttrium, erbium, holmium, zirconium, chromium or other high temperature oxides. When subjected to thermal energy, the fiber layer emits radiation that is directed to a central axis running along the annular passage of the cylindrical burner.
As the ceramic burner is heated, the superemissive fiber layer emits radiation that can be filtered, by use of a cylindrical hollow filter disposed within the ceramic burner, to emit radiation at a selected bandwidth. The radiation passing through the filter is directed to the surface of an optical cable that is disposed centrally within the annular passage of the ceramic burner and filter. The photons generated by the fiber layer are, therefore, directed onto the optical cable and are channeled through the cable to each cable end, which is directed to a target comprising a photovoltaic cell. Accordingly, the photons generated by the superemissive fiber layer within the ceramic burner are directed through the optical cable into the photovoltaic cell, and converted to electricity.
Other embodiments of the photon generator are disclosed in the above-identified U.S. patents and U.S. patent application, which are hereby incorporated by reference. However, each embodiment of the photon generator is similar in that each construction comprises the use of an outer body that is subjected to thermal energy and that includes a superemitter material disposed thereon to effect emission of radiation, i.e., the generation of photons onto a portion of an optical cable used to collect and channel the photons to a photovoltaic target.
The embodiments of the photon generator disclosed in the above-identified U.S. patents and U.S. patent application do not promote the efficient generation and collection of photons. In the above-discussed embodiments, the emission of photons is effected through application of thermal energy to the body, which passes through the body by thermal conduction to the superemissive material. Accordingly, a large amount of thermal energy is wasted through the mechanism of thermal conduction. Additionally, the photons emitted by the superemissive material are not collected efficiently, as relatively weaker photons, or photons that are emitted from a distance further away from the optical cable than other emitted photons, are not collected. Accordingly, all of the photons generated by the superemitter are not collected by the optical cable and, therefore, are not directed to the photovoltaic cell for conversion to electricity.
Additionally, the embodiment of the photon generator described above results in the target, i.e., photovoltaic cell, being subjected to thermal heating via conductive heat transfer from the thermally heated body to each end of the optic cable and to the photovoltaic cell. The thermal heating of the photovoltaic cell commonly leads to thermally related failure due to thermal stresses and thermal shock that develops within the cell.
It is, therefore, desirable that a device be constructed to facilitate the economic generation and collection of photons. It is desired that the device be simple to construct and be easily adaptable to use with a number of different thermal generation sources. It is desirable that the device be configured to accommodate use with one or more photovoltaic cells to effect conversion of photons to electricity. It is desirable that the device be constructed in such a manner as to eliminate or minimize the effects of thermal shock on the cell. It is also desirable that the device be capable of being manufactured from conventional materials using conventional manufacturing techniques.